Reduction of Calcination Temperature in the Calcium Looping Process for CO₂Capture by Using Helium: In Situ XRD Analysis

Abstract

Limestone (CaCO 3 ) calcination to yield CaO plays a central role in a myriad of natural and industrial processes, among which the recently emerged Calcium Looping (CaL) process to capture CO 2 is gaining a great relevance in the last few years. A main drawback of this process, however, is that calcination to regenerate the CaO sorbent particles must be necessarily carried out in short residence times and under high CO 2 partial pressure in order to extract a highly concentrated CO 2 stream from the calciner reactor. This requires raising up the calciner temperature, typically over 930 °C, which brings about an important energy penalty to the technology. Calcination can be speeded up by superheated steam through a chemical action involving H 2 O adsorption, but this catalytic effect leads also to excessively friable CaO solids. This poses an inconvenience for their transport in practice using circulating fluidized bed reactors, since the very fine particles that result from fracturing cannot be recovered by commercial cyclones. The in situ XRD analysis reported in this work shows that, by using helium even at a relatively small concentration, the calcination rate of limestone is notably enhanced even at high CO 2 partial pressures, as corresponds to CaL conditions, whereas the structure and reactivity of the generated CaO remain unchanged as compared to calcination using other balance gases such as N 2 or O 2 . This would allow reducing significantly the calciner temperature at CaL conditions, thus mitigating the energy penalty for CO 2 capture.